[Editor's note: a recurring
subject of debate is why the USAAF and USN relied almost exclusively on
the .50 Browning MG as an aircraft gun during the Second World War, and
did not make more use of the 20 mm Hispano cannon which was in
production in the USA during this period. Some people assume from this
that the .50 Browning was inherently superior to any and all aircraft
cannon of 20+mm, despite the fact that all other major powers engaged
in the war had adopted 20 mm cannon as the principal fighter weapon
long before the end of the war, with some moving up to even larger
calibres. The answer that the explanation lay in production quality
problems with the American Hispano is often greeted with incredulity;
after all, wasn't American industrial production the best in the world?

The history of the acquisition and development of the American Hispano is dealt with at great length in the classic work, "The
Machine Gun", written by George M. Chinn, a retired USMC Colonel, in the early 1950s. What follows is two extracts from this
account, which between them explain (almost) what happened, and why.]

Modifications and Attempts at Standardization

By the time production really began with all
companies, the United States Navy was the only branch of the service
actually placing Hispano Suiza cannon in planes in great numbers and
demanding that aircraft designs of the future include 20-mm in lieu of
lighter machine gun armament. Unfortunately, the gun, which was being
delivered in prodigious numbers, was not proving itself totally
reliable in America.

On the other hand, promising reports were coming
in daily from the British, who had used practically the same procedure
as our Army ordnance engineers in getting the gun into production
status. The original Birkigt Type 404 gun was used as the model for the
Mark I. This was followed shortly by the Mark II. Drawings and a sample
of this improved cannon arrived in the United States for purposes of
study and test in January 1942. The British strongly suggested that
American ordnance officials confer with their representatives in order
to accomplish an early standardization. It was further desired that all
20-mm aircraft cannon of this design procured for British use be of the
Mark II type, the principal differences between the Mark II and the
American M1 being pointed out as follows:

"1. The magazine carrier of the Mark II gun
had a different latch; the ejector was provided with a buffer, and
changes had been made in the magazine holding boss.

2. A heavier rear buffer was provided in the
Mark II gun and the back plate was dovetailed into the receiver instead
of being fitted with a simple groove as in the M1 gun. Inertia blocks
were used in the breechblock slides of the Mark II gun.

3. The sear of the Mark II gun had been modified.

4. Triple wire driving spring and extractor
springs were used in the Mark II gun instead of simple single-strand
coil springs which were used in the M1 gun.

5. Minor changes had been made in the muzzle brake of the Mark II gun.

6. The receiver of the Mark II gun was
substantially different from the one used in the M1 gun; much heavier
guide rails were used and the receiver itself was larger, heavier, and
designed in accordance with British manufacturing methods.

7. The chamber of the Mark II gun was 2 mm shallower than the chamber of the M1 gun."

Practically all the changes suggested were of a
minor nature and slight modifications or alterations would permit
complete standardization. The main difference in the two types of gun
was in the chamber dimensions. Since both were designed to use the same
cartridge, it was quite obvious that one size would best handle the
round. The British were very insistent that their measurements were
better, pointing out, in particular, that their chamber was slightly
more than one-sixteenth inch shorter than the American one. In their
opinion, such a length would solve the problem of faint strikes, since
the weapon was inertia fired and depended upon the shoulder of the
chamber to offer resistance and position the cartridge.

Tests were conducted at Aberdeen, Eglin Field,
Wright Field, and Kenvil Proving Grounds to determine the relative
merits of the British suggestions. The Army Ordnance engineers were not
convinced by these tests that the British chamber was superior to the
American design. However, it was agreed on 4 April 1942 that additional
trials be initiated for the purpose of reaching a satisfactory
compromise for both governments.

The only official action finally taken by the American representatives and approved by the Ordnance Committee was:

"(I) That the manufacture of the American
20-mm automatic gun Ml and AN-M2 be continued in the United States
without modification to the chamber.

(2) That no chamber with the small cone moved
one-sixteenth inch to the rear (as was done in the British chamber) be
considered for manufacture in the United States.

(3) That the request made by British
representatives that 20-mm automatic guns produced in the United States
for British use be made with British chambers not be considered until
after the 20-mm automatic guns Ml and M2 have been subjected to a
thorough test in Great Britain."

After further comparative tests in late April
1942, it was again definitely decided by the Ordnance Department that
all American-made 20-mm automatic guns continue to be made with the
chambers longer by one-sixteenth inch than the British regardless of
the employment of the same ammunition. This decision was final as far
as American production was concerned, but in no way did it change the
British representative's view on the longer chamber's performance.

[In 'Guns of the Royal Air
Force 1939-1945' by G.F. Wallace - who was there - there is an account
of British tests of the British and US Hispanos which took place early
in 1942. The British were unhappy with initial supplies of the
American-made guns: "there
were frequent misfeeds and lightly struck cap stoppages, and the life of several small components was very short" so a comparative test
between one British and three American guns was set up. The intention was to fire 5,000 rounds from each gun without replacing
any components. "The British gun fired the full programme but the performance of the American guns was so bad that in each case the
trial had to be abandoned before the 5,000 rounds had been fired."
The British gun experienced 19 stoppages in firing 5,012 rounds. The
American guns experienced 67 stoppages out of 4,092, 97 out of 3,705
and 94 out of 2,610 respectively. Incidentally, Wallace states that the
US guns were "beautifully made and better finished than our own" and
expressed surprise that although lightly struck caps were a major
source of stoppages, even more frequent were mis-feeds.]

Oddly enough, the question was again raised, not
by the English or our many proving grounds, but by manufacturers of
20-mm ammunition. In testing their cartridges for reliability of
action, they encountered a series of malfunctions known as light-struck
primers that were all out of proportion for such a weapon. These were
not isolated cases, the reports coming in from practically every maker
of 20-mm ammunition that was engaged in function firing his products.

Since the munitions companies pointed out that
the faint strikes were due to lack of impact on the primer resulting
from error in the gun, and not as a result of defective materials or
workmanship, it was decided to conduct another test on an extensive
scale at Aberdeen. Ninety of the 20-mm guns, Ml and AN-M2, selected
from every facility producing them, were expended in this test with all
types of ammunition, both from accepted and rejected lots.

A complete record was made of every malfunction
during the entire test and the probable causes of the trouble. The
engineers in charge of the project in the early stages of this test
recommended that two modifications should be made to overcome the
serious malfunctions:

"(1) Shorten the chamber one-sixteenth inch, thus modifying it to approximately the British chamber.

(2) Replace the extractor spring with a solid
plug, thus positioning the rounds by means of the extractor. This
change would include such modifications to the extractor, the bolt, and
the ejector, as were deemed necessary."

The test began in June 1942 and continued until
the last of January 1943. The final recommendations from Aberdeen
Proving Ground were presented at a meeting attended by representatives
of the Ammunition Branch, Industrial Division, Artillery Branch,
Technical Division, and Field Service. All present accepted as official
four much-needed modifications that were to be made on all 20-mm M1 and
AN-M2 cannon: (1) The chambers were to be shortened one millimeter or
approximately one thirty-second inch; (2) the extractor spring would be
of the cantilever type; (3) the standard firing pin was to have one
sixteenth inch removed from the back of the key slot to give it
"float"; and (4) the breechblock slide springs would be strengthened.

This sanctioned change found the Army with
40,000,000 rounds of ammunition already stocked. While 56,410 guns had
been manufactured to date, it would be easy to make external changes
such as with the firing pin and extractor spring. Barrel chamber
shortening, however, was a problem that generally cost as much in time
and money as to make the whole barrel, to say nothing of the number of
guns immobilized while the modification process was being performed.

Action was taken immediately by the Industrial
Division to put the alterations into effect. There still remained
certain differences between the British Mark II and the AN-M2. As the
British Ministry of Aircraft Production had long advocated having both
guns manufactured identically, the Army Ordnance Department ordered a
comprehensive test in England as soon as the modified weapons came off
the assembly line.

At the suggestion of Capt. E. R. S. Adams of the
British Air Mission, two guns each from International Harvester,
Oldsmobile, and Bendix were shipped to England for the purpose of
competitive aerial tests with the Mark II. Representatives of the Army
Ordnance Department were present to observe the 2,000-round tests which
were held during July and August 1943.

Two British Mark II's were mounted in the left
wing of a Hurricane fighter with two AN-M2's made by Oldsmobile and
International Harvester in the right wing. Combat flying, dives,
G-loading, straight-away, etc., were simulated. One stoppage was
attributed to the Oldsmobile gun. The International Harvester weapon
had no stoppages but a cracked breech-block was noticed at the
completion of the trial. Each Mark II had one sear failure and one of
them had a cracked breech lock after 1,400 rounds. The Bendix guns were
fired on the ground in competition with the British-made guns and made
a creditable showing.

In reporting the findings of the test, Mr.
Hansen, of the British Ministry of Aircraft Production, declared:
"American guns are as good as British guns and are acceptable for
service use".

[There is an inconsistency
here. As will be seen below, the performance of the American Hispanos
remained unsatisfactory throughout the war, yet the British expressed
no concerns and were obviously happy with their guns. There would seem
to be only two possible explanations; either the British tolerance of
unreliability was considerably greater than the American, or the
American guns had been assembled from selected components and thereby
performed better than average.

Wallace states that
although thousands of American-made Hispanos were supplied under
Lease-Lend to the UK, none was ever installed in RAF aircraft. Some
were modified for use as AA guns by shortening the chamber and fitting
triple-wire recoil springs, but these non-standard guns were never used
in operations.

However, the news from America wasn't all bad. Wallace states that: "we
had continual trouble with the recoil units of the British Hispano 20mm
gun - the Mark V gun in particular never worked satisfactorily with the
British designed unit. The American Edgewater Mounting unit...was tried
on the Mark V gun and was found to be a great improvement...In
consequence all Mark V Hispanos were fitted with the Edgewater design
of Front Mounting Unit."]

Performance of Hispano-Suiza Cannon During World War II

It soon became apparent that the Navy would be
the largest user of the 20-mm cannon; in fact, records show that this
branch of the service mounted over 90 percent of the cannon actually
placed in American aircraft. The first 20-mm Hispano-Suiza automatic
gun in a mock up by the Navy was installed at the Bureau of Aeronautics
test facility, then known as the Aircraft Armament Unit, Norfolk, Va.,
on 11 March 1942.

There was nothing slow about the Bureau of
Aeronautics on armament decisions, for the gun was officially accepted
for aircraft use the next day. The installation was in the wing of an
SB-2C which had been shipped separately by the Curtiss-Wright
Aeronautical Corporation, Columbus, Ohio, in order to proof fire the
new cannon.

The Aircraft Armament Unit continued to test the
weapon in its mock-up mounting and it was as late as August 1943 before
the guns were actually placed in the planes at the factory, SB-1C,
serial number 200, being the first to be so armed. It was quickly
followed by plane number 50, SB-2C.

This aircraft, the scout bomber type, proved to
be one of the most widely used during the war and the resulting damage
inflicted upon the enemy by Naval pilots was tremendous. However, the
records reveal such destruction was done by dive bombing rather than by
the use of the 20-mm cannon. The SB-2C first reached the combat area on
11 November 1943 and its first action took place in March 1944.

As an innovation in Navy ordnance, factory
representatives accompanied the new cannon to the front. These expert
technicians sent back voluminous reports that explained that the
malfunctions that did occur were due to one of three things: failure of
the feeder, bad ammunition, and improper maintenance. Their zeal in
clearing the gun itself in every instance casts doubt on the validity
of the reports.

Available records show that the AN-M2 was
installed in 5,800 Naval planes requiring the total mounting of 11,600
guns. The SB-2C and SB-W aircraft were the principal planes carrying
this weapon into combat, along with a very limited number of F4U-1Cs.
Statistics on enemy aircraft shot down in World War II credit the AN-M2
in SB-2C aircraft with destroying few enemy aircraft. The F4U-1C planes
brought down an even smaller number. However, it must be remembered
that the primary mission of the SB-2C was not to shoot down aircraft.

[The utilisation figures given in
this section cannot be correct, at least as far as WW2 production is concerned.
Some 7,200 SB-2C/W were built, using a total of 14,400 cannon. In addition, over
1,400 F6F-5N night fighters, which each carried two cannon, were made. Finally
some 200 F4U-1C were equipped with four cannon. This gives a total of around
18,000 guns used by the USN.

The figure of 90+% of production to
the USN also cannot be right. The USAAF principally used the 20mm Hispano
in two aircraft: the P-38 (over 9,000 built, carrying one cannon each) and the
P-61 (650 built, with four cannon), giving a total of nearly 12,000 guns. Nearly
30,000 guns were therefore fitted to aircraft, of which the USN made use of just
over 60%.

The United States Navy has always permitted the
introduction of evidence even when contrary to what it would like it to
be. There existed two distinct schools of thought on the reliability of
the gun. One was that the 20-mm Hispano-Suiza automatic cannon could
not be considered satisfactory as an aircraft weapon as long as it was
necessary for the ordnanceman to coat the cartridge case with a heavy
lubricant or wax. The other was that this was unimportant as long as it
bettered the performance of the weapon. But everyone even remotely
connected with weapon development agreed on one thing, namely, that 20
millimeters was the minimum caliber for aerial warfare.

[There is another puzzle
here. The RAF were totally opposed to having to wax or oil cartridges,
so according to a primary British source the cartridge was modified so
that such oiling or waxing was unnecessary (unfortunately, the source
doesn't say what changes were made). This is corroborated by the
official RAF Hispano manual, which explicitly forbits oiling the
ammunition. Yet all US Hispanos relied on oiled or waxed ammunition. So
how could the British and American guns and ammunition have been
compatible, as was supposed to be the case? The most likely explanation
is that the British did indeed settle for a hard wax coating, applied
at the factory.]

During war all that can be done is to install and
make function as reliably as possible that which is issued. With the
mounting of the 20-mm cannon in Navy planes a series of malfunctions
began that could not be properly corrected at the time because
manufacture was at the peak of production. The slightest change would
practically mean retooling. The most serious problem was the oversize
chamber. There still remained considerable variance in dimensions
between the chambers of the British and American cannon, even after the
latter chamber was made one thirty-second inch shorter.

Due to an outmoded agreement of long standing,
everything above caliber .60 in the Army is considered artillery and
the manufacture of the Hispano-Suiza cannon therefore came under this
classification. In other words the production of this high-speed
machine gun was done under artillery manufacturing tolerances. The
resulting poor mating of parts, coupled with the inherent fault of all
gas-operated weapons whereby the breech locking key in the receiver is
immovable and the position of the gas port in the barrel is permanently
fixed, made it impossible to adjust the relationship between barrel and
breech lock to establish head space. Thus the most vital measurement in
any automatic weapon was governed by chance in this instance.

An unfortunate discovery was that chamber errors
in the gun could be corrected for the moment by covering the ammunition
case with a heavy lubricant. If the chamber was oversize, it served as
a fluid fit to make up the deficiency and, if unsafe head space existed
that would result in case rupture if ammunition was fired dry, then the
lubricant allowed the cartridge case to slip back at the start of
pressure build up, to take up the slack between the breech lock and the
breech lock key. Had this method of 'quick fix" not been possible, the
Navy would have long ago recognized the seriousness of the situation.
In fact, this inexcusable method of correction was in use so long that
it was becoming accepted as a satisfactory solution of a necessary
nuisance.

This state of affairs continued until the war's
end, at which time all complaints and suggested improvements were
carefully evaluated by the Chief of the Bureau of Ordnance, and a
letter outlining past faults and suggesting improvements was dispatched
on 26 December 1945 from that authority to the Army's Chief of Ordnance
under the subject, "Reactivation of Certain 20-mm Automatic Gun
Development Projects and 20-mm Ammunition Development Projects-Request
for." The following paragraphs are quoted from the letter:

"There is a firm requirement on the part of
the Navy Department for use of 20-mm automatic guns in practically all
Navy combat aircraft currently in design and currently designated
combat operational aircraft -

The 20-mm automatic guns M2 and M3 in their
present stage of development have certain objections and defects which
make continued development of this type weapon highly desirable. The
following features are considered objectionable and are believed
capable of improvement:

(a) The profile of the gun is too bulky for proper installation in VF type wings.

(b) The cyclic rate of the gun is too low.

(c) The belt pull is too low.

(d) It is believed that the over-all weight of the gun and its associated equipments can be materially reduced.

(e) The accumulated tolerances in the
manufacture of the weapon are too great to give uniformly efficient
operation in these guns.

Other objectionable features which are believed capable of rectification are listed below:

(a) The need to oil the ammunition prior to
loading for use in this weapon is undesirable. Self-lubricated
ammunition, or the elimination of the need for lubrication, is strongly
desired.

(b) The ballistics of the projectile can stand
much improvement. It is believed that ballistics similar to that of the
Caliber .60 projectile can be closely approximated.

(c) It is believed that an electrically primed round can be developed for the gun which will give more efficient performance.

(d) The ammunition should be manufactured to
fit the chamber of the gun in which it will be fired and not to fit two
of these weapons - namely, the American and British 20-mm automatic
guns.

From the above, it can be seen that the Navy's
need for improvement in the gun and ammunition is immediate and will be
continuing until the Army's long range development program of an
optimum gun for aircraft materializes. It is understood that the
optimum gun will require from 15 to 25 years for development to be
completed. Continued improvement in the present cannon will certainly
contribute materially in experience gained to the development work
leading toward the optimum gun....

Inasmuch as the Chief of Ordnance is
definitely interested in this development program, this Bureau wishes
to indicate its active interest in and requests that the following
program be undertaken:

(a) Improve the present 20-mm Automatic Gun M3 for immediate needs.

(b) Continue development projects of such guns
as the 20-mm T32 and T33 to arrive at a reliable lightweight, high
performance gun within the next four to six years.

(c) Improve the ammunition for these guns in order to achieve a family of matched projectiles of relatively high performance.

(d) Through the experience gained in this
development program obtain information, data and experience which,
combined with current gun research for an optimum gun, might materially
aid in the development of an optimum gun for aircraft within the next
ten to fifteen years.

To support such a program, this Bureau will
initiate projects complementary to those undertaken by the Ordnance
Department (ASF) to provide competent and experienced personnel and
afford Navy Ordnance facilities to assist in the program. In addition,
this Bureau will furnish funds to support a proportional part of this
development program as established by the estimates of the Chief of
Ordnance."

This letter resulted in the cooperation of the
Army, with Navy engineering personnel, familiar with the conditions
that needed remedying, in solving the various problems. Today, barely
five years after the war, every point brought out by the Navy's Chief
of the Bureau of Ordnance has been answered. Nothing was basically
wrong with the weapon. Its wartime performance, good or bad, was the
result of having been bought in desperation, put into mass production
without first having been adequately proved, and then modified
regularly to meet a future commitment before the previous model had
been made to function reliably.

[Chinn goes on to describe
the postwar development of the American Hispano, implying that all of
the problems were solved within a few years in the much-modified M3 and
M24 variants. However, a separate source mentions that when the French
received a batch of F8F-1B Bearcat fighters for use in SE Asia in the
early 1950s, they found that a high proportion (25 out of 64) of the
installed 20 mm M3 cannon were defective, due to parts being outside
the specified margins, and had to be replaced].